[1011.2729]
Have Baryonic Acoustic Oscillations in the galaxy distribution really been measured?

Authors:

Anna Cabre, Enrique Gaztanaga

Abstract:

Recent publications claim that there is no convincing evidence for
measurements of the baryonic acoustic (BAO) feature in galaxy samples using
either monopole or radial information. Different claims seem contradictory:
data is either not consistent with the BAO model or data is consistent with
both the BAO model and featureless models without BAO. We investigate this
point with a set of 216 realistic mock galaxy catalogs extracted from MICE7680,
one of the largest volume dark matter simulation run to date, with a volume of
1300 cubical gigaparsecs. Our mocks cover similar volume, densities and bias as
the real galaxies and provide 216 realizations of the Lambda or w=-1 Cold Dark
Matter (wCDM) BAO model. We find that only 20% of the mocks show a
statistically significant (3 sigma) preference for the true (input) wCDM BAO
model as compared to a featureless (non-physical) model without BAO. Thus the
volume of current galaxy samples is not yet large enough to claim that the BAO
feature has been detected. Does this mean that we can not locate the BAO
position? Using a simple (non optimal) algorithm we show that in 50% (100%) of
the mocks we can find the BAO position within 5% (20%) of the true value. These
two findings are not in contradiction: the former is about model selection, the
later is about parameter fitting within a model. We conclude that current
monopole and radial BAO measurements can be used as standard rulers if we
assume wCDM type of models.

There is a claim by Gaztañaga et al in 0807.3551 to have detected the radial mode of the BAO signal, and in 0808.1921 the signal was used it to put constraints on cosmological parameters. This would be important, because the radial signal provides a measurement of H(z) which is independent of distance measurements, making it possible to test the FRW metric (see 0712.3457).

Kazin et al analysed the radial signal in 1004.2244. They found that although there is a clustering signal in the data, it is not possible to distinguish the model with BAO from a model without BAO with statistical significance. They concluded that therefore it cannot be concluded that the clustering feature would be due to the radial BAO.

Now Cabré and Gaztañaga return to the topic. They agree that the models with and without the radial BAO signal cannot be distinguished by the current data. However, they find that the same is true for the BAO monopole signal: there is no detection of the BAO. (See also 0903.0950, where it is also argued that the BAO have not been detected.) Nevertheless, they argue that if you assume that the features seen in the data are the BAO and not noise, you can still get good parameter constraints.

I would compare this to seeing a peak in particle physics experiments. If you assume that the peak corresponds to a real particle, then you may be able to determine the mass very accurately, if the peak is very sharp. However, this says nothing about the probability that the peak may be just noise. However, I would somehow think that the error bars should include the possibility that the feature is noise. As noted, this holds true also for the usual BAO analysis.